I verified that just about 13 dB down on the first sidelobe gives you
the best pattern, and is about .2 down from highest gain.
tom
K0TAR
Tom Ring wrote:
Or you can do it using YO, EZNEC, etc., and increase the spacing until
the first sidelobe is minimized, and the next stacking distance step
makes it larger. I think that's at 13dB down, but I could be wrong,
17dB also sounds correct. 
tom
K0TAR